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Re: [AUDITORY] AC/BC: Why would bone conduction be poorer than air conduction?



I have found that usually bc vs ac result problems have either to do with poor
bone vibrator placement or with a need to clean either the vibrator surface
and/or the skin over the mastoid process.

Having said that, I have seen a few cases in clinic over the years where bc
scores simply made no sense and have either discovered a longitudinal fracture
in the patient's history or could take care of the problem with vibrator
placement at another site or from another audiometer.

I apologize for my typing -- have had shoulder surgery and fingers don't work
quite like they're supposed to any more.

Tom


Tom Brennan  KD5VIJ, CCC-A/SLP
web page http://titan.sfasu.edu/~g_brennantg/sonicpage.html

On Thu, 18 Feb 2016, Jan Schnupp wrote:

> Date: Thu, 18 Feb 2016 11:14:56 +0000
> From: Jan Schnupp <jan.schnupp@xxxxxxxxxxxxx>
> To: AUDITORY@xxxxxxxxxxxxxxx
> Subject: Re: AC/BC: Why would bone conduction be poorer than air
>     conduction?
>
> Dear Becky,
>
> I would be strongly inclined to think that the variabilities in BC with
> frequency and placement and other parameters you see in some way or another
> boil down to acoustic impedance mismatches. Vibrations from your BC
> oscillator somehow have to travel through skin then bone to the ear, and at
> each interface there is the potential for more or less of the sound energy
> to be reflected rather than transmitted, and this is likely to be frequency
> dependent. You say you see unexpectedly poor thresholds mostly at lowish
> frequencies. That sounds like frequency dependent impedance to me. The
> slower the vibration, the less the tiny little bone conductor has to make
> much of an impression on the much larger, heavier, hard, bony head,
> particularly when coupled through a layer of elastic skin. If you want to
> get a good intuitive feel for acoustic impedance, what it is, why it is
> important generally and also particularly for the question you are asking,
> I would recommend you try to get hold of a copy of Eric Heller's very fine
> book "Why you hear what you hear". One of the most accessible and clear
> introductions to physical acoustics I have come across.
>
> All the best,
>
> J
>
>
>
> On 18 February 2016 at 00:05, Becky Lewis <becky103@xxxxxx> wrote:
>
> > Hello all,
> >
> > In general, when we hear using bone conduction (BC), we should expect to
> > hear the same or better than when we hear using air conduction (AC) due to
> > the physical properties of the ear. With poorer BC thresholds, generally
> > the culprit that is offered in clinic is poor bone oscillator placement.
> > However, there are patients who demonstrate BC thresholds that are up to
> > 30dB poorer than AC thresholds at 0.5kHz in particular, which placement
> > would not account for alone. Other frequencies do not produce this same
> > effect. Additionally, movement of the oscillator can result in no change in
> > this AC/BC difference.
> >
> > Aside from bone oscillator placement, are there other reasons that could
> > produce a BC threshold at 0.5kHz that is 20-30dB worse than AC
> > threshold? I've started to consider variability in bone density, force of
> > the oscillator on the temporal bone (Toll et al., 2011), the differences in
> > properly calibrated oscillators... I am open to any thoughts or research
> > articles recommended by this group to assist my finding an answer to this
> > question.
> >
> > Thank you in advance for your assistance!
> >
> > Wishing you all the best,
> > Becky Lewis
> > --
> > Rebecca Lewis, PhC
> > Doctorate of Clinical Audiology (AuD) Student
> > Doctorate of Philosophy (PhD) Candidate
> > Speech and Hearing Sciences, University of Washington - Seattle
> > *Expected Graduation Date: 6/30/2016*
> >
>
>
>
> --
> Prof Jan Schnupp
> University of Oxford
> Dept. of Physiology, Anatomy and Genetics
> Sherrington Building - Parks Road
> Oxford OX1 3PT - UK
> +44-1865-282012
> http://jan.schnupp.net
>